JWST ETC Coronagraphy Strategy

The JWST Exposure Time Calculator (ETC) Coronagraphy strategy is used for calculating the signal-to-noise ratio and contrast for MIRI and NIRCam coronagraphy ETC calculations.

For coronagraphic ETC calculations, a user would be interested in the signal-to-noise ratio (SNR) that can be achieved for a faint companion that is close to a bright star, or for circumstellar features around a bright host star. The bright star in this case can be many orders of magnitude brighter than the faint features of science interest. The ETC provides predictions for coronagraphic observations by including reference PSF stars for PSF subtraction from the source of interest. There are two tabs under Strategy for coronagraphy in the ETC: (1) Observation and (2) Extraction

Figure 1. Layout of the Observation under the Strategy tab for Coronagraphy

(1) Observation: Parameters for the PSF calibration are set under this tab.

  • Scene rotation: This allows rotation of the scene with respect to the non-azimuthally symmetric structure of the coronagraph's focal plane (such as cross pattern for the MIRI FQPM, or bars for NIRCam) . When used in conjunction with the coronagraph visibility tool (at the page JWST Coronagraphic Visibility Tool Help), this feature allows the user to determine the optimal aperture position angle special requirement for the scene of interest.
  • PSF subtraction source:The reference star that should be used for PSF subtraction can be selected from the drop-down menu that has a list of all the sources in the scene.
  • PSF subtraction: The default PSF subtraction strategy is currently set to Optimal. The optimal strategy assumes that thermal and dynamical changes do not occur in the optical system between observations of the science target and the reference PSF star exposures. Residual noise, in this case, is driven solely by the shot noise in the wings of the host star and reference PSF source profiles, and SNR for the detection of the faint source depends on the exposure time. Ideally, the reference PSF star would be brighter and will be re-scaled for PSF subtraction. However, as currently implemented in the ETC, the user must set the magnitude and stellar type of the reference star to closely match that of the target star in order to yield a realistic result. Choosing different magnitudes and/or stellar types for the target and reference star will not trigger a warning in the ETC, but the user should be aware that such results might not be accurate.
    • Since March 2018 (patch release ETC 1.2.2), three more options are available in addition to the default one:
      • Optimal (No PSF Autoscaling): will not scale the PSF subtraction source to match the central source before subtracting. It was the default behavior in v1.1.1 and earlier of the Exposure Time Calculator. 

      • Unsubtracted Science Scene displays only the science scene, with no subtraction and only the coronagraphic mask suppressing the central source. 

      • PSF Subtraction Source Only displays the PSF subtraction source by itself, under the coronagraphic mask.
        Note: contrast and SNR estimates are reported for all options, though they are not particularly meaningful for the Unsubtracted Science Scene or PSF Subtraction Source Only options (particularly the PSF Subtraction Source Only option, where the SNR source is not present). Those two options are primarily provided for users who wish to download (separately) the simulated data and analyze the observations themselves. It is particularly useful, for instance, for users who wish to use a brighter reference PSF star with possibly its own set of readout parameters. 

The Images (2D SNR, Detector, Saturation and Groups Before Saturation) and Reports (Report, Warnings, Downloads) displayed change depending on which PSF subtraction option is selected.

In order to have correct saturation information regarding the science scene and/or the PSF Subtraction Source, the latter needs to be positioned away from the science scene (e.g. by 10 arcsec). This is done via the Offset sub-tab of under Scene or using the Source Editor when defining the scene(s) and sources.

Figure 2. Layout of the Extraction under the Strategy tab for Coronagraphy

(2) Extraction: The details of the extraction aperture and background subtraction are set under this tab.

  • SNR source: From the drop-down menu, which lists all the sources in the scene, the user can choose the faint source for which the SNR has to be calculated. The SNR source must lie within a square, centered on the coronagraphic mask and aligned with the detector rows and columns, with sides of 101 pixels for NIRCam or 81 pixels for MIRI.
  • Aperture radius: The coronagraphy strategy uses a circular aperture and this refers to the radius of the extraction aperture in arcsec.
  • Contrast azimuth: This sets the angle at which the contrast plot will be computed, and is expected to be the angle along which the faint source of interest is located.
  • Contrast separation: This is the separation along the direction specified by the azimuth angle at which the scalar values are reported.

Coronagraphy is the only mode for which the contrast plots are available in the Plots pane.




Published

 

Latest updates

  • Updated for ETC v1.3

  •  
    To reflect changes in the ETC v1.2.2 patch release (PSF subtraction options)